Steve;
I used this on my aluminum panels.
https://www.aircraftspruce.com/catalog/cspages/ptiprimers.php?clickkey=97961
View attachment 94779View attachment 94780You have a choice of yellow (actually greenish-yellow) or green zinc chromate.
First, scuff the surface with Scotchbrite and then treat the surface with a phosphoric acid solution, then rinse thoroughly and let dry. Apply the primer per instructions and you have a nice hard surface that is ready to top coat. I used an aerospace grade isocyanate polyurethane yellow over this primer on my steel tube chassis but I left the aluminum in primer because I liked the "aircraft" look.